Portable communication devices, such as cellular telephones, typically are required to operate over a number of different communication bands. These so called “multi-band” communication devices use one or more instances of transmit and receive circuitry to generate and amplify the transmit signals and amplify the receive signals, respectively. However, these communication devices usually employ a single antenna to both transmit and receive the signals over the various communication bands. Although some systems may include multiple antennas, for example, a high-band antenna and a low/mid-band antenna, at least one of these antennas is still often used to transmit or receive in multiple frequency bands. The antenna in such communication devices is typically connected to the transmit and receive circuitry through switching circuitry, such as a duplexer or a diplexer, or through an isolated switch element, sometimes referred to as a “transmit/receive switch” or an “antenna switch.” The switching circuitry or antenna switch must effectively isolate the transmit signal from the receive signal. Isolating the transmit signal from the receive signal becomes more problematic in a multiple band communications device where the transmit frequency (or a harmonic thereof) of one communication band might overlap with the receive frequency of a different communication band.
Various electronic components used in communication devices include transistors used for various purposes, including switching and amplification. In modern wireless communication devices, linearity and physical size of transistor-based components, including the antenna switching circuitry and low noise amplifiers, for example, are significant design factors. Linearity is usually defined by what is referred to as a third order intermodulation distortion (IMD3). The IMD3 signal may deteriorate the sensitivity of the receiver if the antenna switch or switching circuitry allows a sufficiently high IMD3 signal through. Furthermore, IMD3 performance in the low noise amplifier(s) can also be critical to the sensitivity and overall performance of the receiver.
Conventional approaches to improving IMD3 performance in transistor-based components include increasing the bias current or voltage, increasing the size of the transistors (with an associated increase in the component die size), changing the circuit topology, and using advanced process technology in the fabrication of the devices. However, these approaches have associated drawbacks. For example, as noted above, it is often desirable to minimize the size and power requirements of mobile communication devices, and therefore increasing the die size of components or the bias current/voltage is not necessarily a preferred approach. Similarly, requiring the use of specialized or advanced process techniques can increase the cost of the devices.